This invention relates to a contour correcting circuit for emphasizing the contour of a video signal.
A conventional contour correcting circuit is shown in FIG. 1. In FIG. 1, reference numeral 1 designates a high frequency extraction circuit for extracting a high frequency component from an input video signal; and 2, an addition circuit for adding the output of the high frequency extraction circuit 1 to the input video signal. The high frequency extraction circuit is, in general, a secondary high-pass filter, or a secondary differentiation circuit using a delay line.
In the case where the secondary differentiation circuit is employed as the high frequency extraction circuit 1, a high frequency component is extracted from the video signal (shown in part (a) of FIG. 2) by the high frequency extracting circuit 1 as shown in part (b) of FIG. 2, and is then added to the input video signal by the addition circuit 2, as a result of which an output as shown in part (c) of FIG. 2 is provided at the output terminal.
However, the conventional contour correcting circuit is disadvantageous in that, since the high frequency component extracted from the input video signal is added to the input video signal, the noise is increased and accordingly the S/N ratio is lowered. Furthermore, the contour correcting circuit suffers from a difficulty in that preshooting or overshooting increases the contrast ratio of the contour, thus lowering the picture quality.
In addition, the contour correcting circuit involves a problem in that, if the overshoot is decreased to a level small, it becomes impossible to make the signal's rise and fall characteristics steep.